1. Field of the Invention
The present invention concerns a self-powered sensing module capable of operating without having to use separate power supplied externally and a tire pressure monitoring system using the sensing module.
2. Description of the Related Art
A tire for a wheeled vehicle generally transfers driving power, brake power, and lateral force of the vehicle onto a road while supporting the load of the vehicle against the road, and also serves as a spring and a damper for absorbing shock and impact with respect to the road.
If a tire pressure is above a suitable range, the vehicle tramps over the road to cause ride discomfort and acceleration of the abrasion of the tire. If the tire pressure is below the suitable range, acceleration of the abrasion of the tire occurs as well and this condition may also bring about a traffic accident due to a puncture occurring to the tire during driving. In addition, the driving of the vehicle deteriorates and fuel efficiency decreases. Accordingly, it is essential to maintain the suitable tire pressure.
To address the above issues, a tire pressure monitoring system (TPMS) is under development so as to enable a vehicle operator to monitor the physical state, such as tire pressure and temperature, through a display.
FIG. 1 depicts an example of a conventional TPMS 12. The TPMS 12 includes four pressure sensing modules 15a, 15b, 15c and 15d and a main unit 17.
The pressure sensing modules 15a, 15b, 15c and 15d are mounted in tires 13a, 13b, 13c and 13d, respectively. The main unit 17 is secured in a body of a vehicle 11.
Referring now to FIG. 2, each of the pressure sensing modules 15a, 15b, 15c and 15d includes a pressure sensor 21, a transmission circuit 22, a battery 23, and a transmission antenna 24.
The battery 23 may be a lithium battery and supplies power to the pressure sensor 21 and the transmission circuit 22. The transmission antenna 24 irradiates a radio wave within a radius of the tire 13a, 13b, 13c or 13d. The pressure sensor 21 communicates with the transmission circuit 22 which is connected to the transmission antenna 24 through an output terminal 25.
The pressure sensor 21 detects the pressure in the tire 13a, 13b, 13c or 13d and transmits to the transmission circuit 22 a signal carrying information on the detected pressure.
The transmission circuit 22 is provided with an internal memory (not shown) storing a unique identification (ID) code. The ID code is to identify the pressure sensing modules 15a, 15b, 15c and 15d mounted in the vehicle 11 from each other. The transmission circuit 22 performs modulation in order to generate a signal carrying data indicating the pressure information and the ID code, and transmits the modulated signal as the radio wave over the transmission antenna 24.
Referring to FIG. 3, the main unit 17 includes four reception antennae 31a, 31b, 31c and 31d, a reception circuit 33, a controller 34, and a display 18.
The reception circuit 33 receives via the reception antennae 31a, 31b, 31c and 31d the radio wave from the transmission antenna 24 of the pressure sensing modules 15a, 15b, 15c and 15d. The reception circuit 33 detects and demodulates the radio wave carrying the pressure data, and transfers the demodulated data to the controller 24.
The controller 34 controls the display 18 to display information on the tire pressure based on the received data.
The display 18 is mounted at a cockpit and is connected with the controller 34.
However, the conventional TPMS 12 has a disadvantage that the vehicle operator needs to replace the battery 23 by separating each of the tires 13a, 13b, 13c and 13d when the battery 23 is run down for supplying the power to the pressure sensor 21 of the pressure sensing module 15a, 15b, 15c or 15d and the transmission circuit 22. This is because the pressure sensing modules 15a, 15b, 15c and 15d are mounted in the tires 13a, 13b, 13c and 13d, respectively.
Since the lithium battery 23 is expensive, it costs a lot to maintain the TPMS 12. In addition, the pressure sensing modules 15a, 15b, 15c and 15d, which are mounted in the tires 13a, 13b, 13c and 13d, increase in volume due to the size of the lithium battery 23.
To overcome the above disadvantage, there is provided another TPMS (not shown) which estimates the pressure of the tire by measuring elasticity of the tire from the suspension of the vehicle, instead of measuring the pressure in the tires 13a, 13b, 13c and 13d. 
Such a TPMS overcomes the above disadvantage of separating the tires in order to replay the battery. However, the TPMS has a drawback that it is hard to obtain accurate tire pressure since the tire pressure is estimated based on the elasticity of the tires.